CHAPTER V WATERSHED CHARACTERIZATION USING GIS

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1 61 CHAPTER V WATERSHED CHARACTERIZATION USING GIS 5.1 GENERAL: In this chapter, introduction to watershed characterization and analysis has been presented in the first part. Methodology to compute important characteristics of the watershed namely, Area of watershed, Perimeter, Bifurcation, Elongation, Circulatory, Form factor, Stream order, Drainage density, Average slope of watershed, Main stream channel slope etc. has been discussed in the second part. 5.2 INTRODUCTION: The response of a particular watershed to different hydrological processes and its behaviour depends upon various physiographic, hydrological and geomorophological parameters. Though these are watershed specific and thereby unique, the characterization of a watershed provides an idea about its behaviour. Hydrologists attempted to relate the hydrologic response of watersheds to watershed morphologic characteristics. Computation of watershed morphological characteristics is a prerequisite to further detailed hydrological analysis of the watershed. Watershed characterization involves measurement of parameters that influence the characteristic behaviour of a watershed whereas analysis aims at the critical study of these parameters to arrive at conclusions on watershed response and behaviour. Important watershed morphometric characteristics

2 62 included in the study are: Area of watershed, perimeter, Bifurcation, Elongation, Circulatory, Form factor, Stream order, Drainage density, Average slope of watershed, Main stream channel slope etc. Geographical Information System when used in conjunction with conventional data could provide valuable inputs such as watershed area, size and shape, topography and drainage pattern for watershed characterization and analysis. Watershed boundary map and Drainage network map as shown in Fig 5.1 were prepared and utilized for computation of the morphological characteristics of the watershed in GIS environment. GIS could be effectively used for the computation of these morphological characteristics of the watershed with greater efficiency and accuracy. The watershed has emerged as the basic planning unit of all hydrologic analyses and designs. Watersheds considered in engineering hydrology vary in size from a few hectares in urban areas to several thousand square kilometers for large river basins. Each watershed shows distinct characteristics, which are so much variable that no two watersheds are identical. All the characteristics affect the disposal of water. Certain physical properties of watersheds significantly affect the characteristics of runoff and as such are of great interest in hydrologic analyses. Morphological characteristics like stream order, drainage density, aerial extent, watershed length and width, channel length, channel slope and relief aspects of watershed are important in understanding the hydrology of the watershed. Runoff response of the watershed is different for various

3 63 slopes, shapes, lengths, widths and areas of watershed. Response is also affected by the factors like drainage density, length of overland flow, stream frequency, relative relief and relief s. A detailed analysis of the drainage network in a watershed can provide valuable information about watershed behaviour which will be useful for further hydrological analysis. The order, pattern, and density of drainage have a profound influence on watershed as to influence runoff, infiltn, land management etc. It determines the flow characteristics and thus erosional behaviour (Murthy, 2000). The Geographic Information System (GIS) has unique features to relate to the point, linear and area features in terms of the topology as well as connectivity (Murali Godavari, 2006). Increased interest is being directed to the mapping of hydro-geomorophological characteristics using GIS and Remote Sensing techniques (Epstein et al., 2002). Walsh (1998) described the applications of remote sensing and GIS for geomorphic research. Watershed boundary map, Drainage network map and Contour map were prepared and utilized for computation of the morphological characteristics of the watershed using Arc GIS 9.2. Important watershed characteristics included in the study are discussed in terms of linear, aerial and relief aspects. 5.3 METHODOLOGY A common task in hydrology is to delineate the watershed from a topographic map. The Survey of India topographic maps namely 56-I 3, I 6, I 7, I 8, I 10, I 11, I 12, I 14, I 15 &I 16 on a scale of 1:50,000 were collected. The collected topographic sheets were scanned and

4 64 registered with tic points and rectified in Arc map of Arc GIS 9.2. Further, the rectified maps were projected and merged together as a single layer. The present study area of Kaddam watershed of G-5 sub basin i.e., Middle Godavari sub basin of Godavari River Basin was delineated in GIS environment.stream network of the study area is digitized from SOI toposheets of 1:50000 scale which are geocoded in ERDAS IMAGINE. One of the first attributes to be quantified was the hierarchy of stream segments according to an ordering classification system based on ranking of streams proposed by Strahler (1964). In this system, channel segments were ordered numerically from a stream's Headwaters to a point somewhere down stream. Numerical ordering begins with the tributaries at the stream's headwaters being assigned the value 1. A stream segment that resulted from the joining of two 1st order segments was given an order of 2. Two 2nd order streams formed a 3rd order stream, and so on. The trunk stream through which all discharge of water passes is therefore the stream segment of the highest order.fig.2 shows the Drainage network map of the study area. The number of stream segments present in each order along with their lengths is recorded in the topology built by GIS. Formulae and relationships for the computation of the morphometric parameters are listed in Table 5.1.

5 65 Table 5.1 Formulae and relationships for the computation of the morphometric parameters Morphometric Formula/Relationship Reference parameter Stream order Hierarchical rank Strahler,1964 Stream length Length of stream Horton,1945 Mean Stream Lsm=Lu/Nu,Where, Lu=Total stream Strahler,1964 length length of order u,nu=total no of stream segments of order u Stream length RL= Lu/Lu-1, Where, Lu=Total Horton,1945 stream length of order u, Lu-1=the total stream length of its next lower order. Bifurcation Rb= Nu/Nu+1, Nu= total number of stream segments of order u, Nu+1= number pf stream segments of the Mean Bifurcation Relief Drainage density Stream frequency Drainage Texture Form factor next higher order. Rbm= average of the bifurcation of all order Rh= H/Lb, where H= total relief (relative relief) of the basin, Lb= basin length. D= Lu/A, where A is the total area of the basin (km) 2, Lu is the total stream length of all orders. Fs= Nu/A, Where Nu is the total number of streams of all order, A is basin area in km 2 Rt= Nu/P, where Nu is the total number of streams of all order, P is the perimeter of the basin in km 2 Rf= A/Lb 2 is the square of the basin length (km), A is the basin area in km 2 Circularity Re= 4π A/P 2, where A is the area (km) 2 and p is the perimeter (km) of the watershed Elongation Length of overland flow Re= 2sqrt (A/π )/Lb, where A is the area (km) 2 and P is the perimeter (km) of the watershed Lg= 1/(D*2), where D is the drainage density Strahler,1957 Horton,1945 Miller,1953 Horton,1945